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There have been reported strong phreatic explosions at Guagua Pichincha volcano on February 1st. There was a increase in activity in the last weeks and it is recommended to stay away from the volcano. ...
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Guagua Pichincha is one of Ecuador's most active volcanoes. The stratovolcano is part of a complex that rises immediately west of the capital Quito, at only 8 km distance from the city center. 3 major explosive eruptions have occurred at Guagua Pichincha in the past 2000 years, the most recent one in 1660. An event of similar size today is a major threat to the ca. 2 million city of Quito.
The volcano and the older extinct Rucu Pichincha stratovolcano form a broad volcanic massif 23 km in diameter. Historical activity of the volcano has included large explosive eruptions some of which produced sub-plinian and plinian eruption columns, lava domes. Guagua Pichincha volcano's eruptions frequently produce dangerous pyroclastic flows.
Following a 100 year long interval of being dormant, Guagua Pichincha volcano has entered a new phase of low seismic, phreatic and magmatic activity in 1981.

Introduction:

from Smithsonian / GVP volcano information:A lava dome is located at the head of a 6-km-wide breached caldera that formed during a late-Pleistocene slope failure of Guagua Pichincha about 50,000 years ago. Subsequent late-Pleistocene and Holocene eruptions from the central vent in the breached caldera consisted of explosive activity with pyroclastic flows accompanied by periodic growth and destruction of the central lava dome. Many minor eruptions have occurred since the beginning of the Spanish era at Guagua Pichincha, which is one of Ecuador's most active volcanoes. The largest historical eruption took place in 1660, when ash fell over a 1000 km radius, accumulating to 30 cm depth in Quito. Pyroclastic flows and surges also occurred, primarily to the west, and affected agricultural activity, causing great economic losses.

Plinian eruptions from Guagua PichinchaOver the past 2000 years, 3 Plinian eruptions happened at the Guagua Pichincha volcano, in the 1st century AD, in the 10th century, and in AD 1660. The eruptions produced plinian eruption columns of 25-30 km height and left well-defined fallout pumice layers covering the upper slopes of the volcano. The 10th century eruption was the largest, with a VEI of 5.
(C. Robin et al. (2008) "Late Holocene phases of dome growth and Plinian activity at Guagua Pichincha volcano (Ecuador)", Journal of Volcanology and Geothermal Research, 176, pp. 7–15)

2008 eruptions7 phreatic eruptions occurred at Guagua Pichincha Volcano on 1 February 2008. The eruptions were preceded by a magnitude 4.1 earthquake on 6 December 2007.

2003 possible eruptionOn 17 April 2003, seismic signals were detected that could have accompanied a minor eruption at Guagua Pichincha, but there was no visual confirmation that an eruption happened.

2002 eruptionOn 11 October 2002, 4 phreatic eruptions occurred after several days of heavy rainfall. The eruptions ejected blocks 100-200 m from the vent. Long-period and volcano-tectonic earthquakes and continuous background tremor were recorded until 17 October.
Source: GVP monthly reports

2001 eruption - lava dome growth and ash explosionsGuagua Pichincha's activity in early 2001 consisted principally of gradual lava dome growth. There were ash eruptions on 18, 31 March, and 25 May 2001. The explosion on 25 May produced a ash cloud reaching an altitude of 8.5 km. On 26 November 2001, seismic signals indicated a 20 minute-long phreatic explosion around noon, followed by 16 hours of continuous tremor.

2000 eruptionsDuring 2000, Guagua Pichincha produced a series of mostly minor ash explosions. A partial collapse of its lava dome which grew in December 199 and January ("dome 8") occurred on 12 July.

1997 phreatic eruptionsDuring March-October 1997, Guagua Pichincha produced a series of phreatic explosions. The eruptions occurred without any precursory warning signal. Their intensity peaked in May 1997 and the last one was recorded on 18 October 1997. This activity resembled phreatic explosions that occurred in 1981, 1990, and 1993.

1993 eruptions and fatalitiesA phreatic explosion at 11h46 local time on 12 March 1993 from Guagua Pichincha's central dome killed 2 young volcanologists. A warning of possible activity had been transmitted to them by radio at 10h30, but for unknown reasons they were still on the lava dome when the eruption occurred.
(source: GVP monthly reports)

1981 reawakening after 100 years of slumberNew activity started in 1981 with phreatic ash explosions and an increase in fumarole activity. This was the first activity after about 100 years of dormancy and started a new cycle of activity.

1660 Plinian eruption of Guagua PichinchaQuito has been significantly affected several times by eruptions of Guagua Pichincha. The worst of these events occurred in 1660, when as much as 25 cm of ash covered the city. A similar event today, as the its population approaches 2 millions, could spell a major disaster.
The eruption in 1660 began on 28 October 1and lasted for 12 hours. A plinian eruption column developed, reached ca. 25 km altitude and deposited ash and pumice over Quito, which was in a total darkness during the eruption. Pyroclastic flows and surges swept down the flanks of the volcano, but the topographic relief of Rucu Pichincha prevented flows from reaching Quito itself.
The ash fell as far north as Popayan, Columbia (300 km ), and to the south at Loja (430 km ), and to the west it reached the Pacific coast (170 km). Earthquakes during the eruption caused panic in Quito.
There are no reports of fatalities, but the the eruption caused a significant economic downturn to the city and its surrounding region.